Marking paper products

ABSTRACT

Methods of marking paper products and marked paper products are provided. Some methods include irradiating the paper product to alter the functionalization of the paper.

RELATED APPLICATIONS

This application is a continuation application of U.S. Ser. No.16/047,155, which is a continuation application of U.S. Ser. No.15/141,679, filed Apr. 28, 2016, now abandoned, which is a continuationapplication of U.S. Ser. No. 14/308,190, filed Jun. 18, 2014, now U.S.Pat. No. 9,342,715, granted on May 17, 2016, which is a continuationapplication of U.S. Ser. No. 13/440,141, filed Apr. 5, 2012, now U.S.Pat. No. 8,986,967, granted on Mar. 24, 2015, which is a continuation ofInternational Serial No. PCT/US2010/052388, filed Oct. 12, 2010, whichclaims priority of U.S. Provisional Application Ser. No. 61/251,633,filed on Oct. 14, 2009. The entirety of each of these applications areincorporated herein by reference.

TECHNICAL FIELD

This invention relates to methods and systems for marking paperproducts, such as currency, and products produced by such methods andsystems.

BACKGROUND

Paper, as that term is used herein, refers to the wide variety ofcellulose-based sheet materials used for writing, printing, packaging,and other applications. Paper may be used, for example, but withoutlimitation, in the following applications: as paper money, bank notes,stock and bond certificates, checks, postage stamps, and the like; inbooks, magazines, newspapers, and art; for packaging, e.g., paper board,corrugated cardboard, paper bags, envelopes, wrapping tissue, boxes; inhousehold products such as toilet paper, tissues, paper towels and papernapkins; paper honeycomb, used as a core material in compositematerials; building materials; construction paper; disposable clothing;and in various industrial uses including emery paper, sandpaper,blotting paper, litmus paper, universal indicator paper, paperchromatography, battery separators, and capacitor dielectrics.

In some applications, for example when paper is used as currency and inother financial applications, it is often desirable to be able to “mark”or “tag” the paper with a special marking that is not visible to thenaked eye, and/or cannot easily be produced by counterfeiters. Markingcan be used, for example, to prevent or detect counterfeiting ofcurrency, art and other valuable documents. Marking can also be used oncurrency to allow the currency to be traced and/or identified, e.g., ifit is stolen or used in a criminal transaction.

SUMMARY

The invention is based, in part, on the discovery that by irradiatingpaper at appropriate levels, the functionalization of the irradiatedpaper can be altered, thereby making the paper distinguishable, e.g., byinfrared spectrometry (IR) or other techniques, from paper that has notbeen irradiated. In some cases, the paper is also distinguishable frompaper that has been irradiated, but under other process conditions. As aresult, paper products such as currency can be “marked” by the methodsdescribed herein. In some implementations, the marking is invisible tothe naked eye, e.g., it is detected by the use of instruments. In otherimplementations, the marking is visible to the naked eye. Generally, themarking is difficult to replicate without relatively sophisticatedequipment, thereby making counterfeiting more difficult.

By “functionalization,” we mean the functional groups that are presenton or within the paper.

In one aspect, the invention features methods of making a marked paperproduct. Some methods include irradiating a paper product underconditions selected to alter the functionalization of at least an areaof the paper product.

Some implementations include one or more of the following features. Thepaper can be irradiated with ionizing radiation. The dose of ionizingradiation can be at least, for example, 0.10 MRad, e.g., at least 0.25MRad. The dose of ionizing radiation can be controlled to a level ofabout 0.25 to about 5 MRad. Irradiating can include irradiating withgamma radiation, and/or with electron beam radiation or other particles.Electrons in the electron beam can have an energy of at least 0.25 MeV,e.g., from about 0.25 MeV to about 7.5 MeV.

The methods can further include quenching the irradiated paper product.For example, quenching can be performed in the presence of a gasselected to react with radicals present in the irradiated paper product.

In some cases, only a portion of the paper product is irradiated. Insome cases, only a portion of the irradiated area, or only a portion ofthe paper product as a whole, is quenched. For example, an area that isto remain unmarked and/or unquenched can be masked.

Irradiation can occur during formation of the paper product. Formationcan include amalgamating the pulp material into a wet paper web.Irradiating can be performed on the wet paper web or prior to formationof the wet paper web. Formation can further include drying the wet paperweb, and irradiating can occur after drying. In some implementations,powders, granulates, chemical solutions, dyes, inks, or gases can beapplied, singularly or in combination, before, during, or afterformation of the paper.

In another aspect, the invention features marked paper products thatinclude a cellulosic or lignocellulosic fibrous material containingfunctional groups not present in a naturally occurring cellulosic orlignocellulosic fibrous material from which the marked paper product wasobtained.

The cellulosic or lignocellulosic material in the paper product can beselected, for example, from the group consisting of fiber derived fromwood and recycled paper, vegetable fiber materials, such as cotton,hemp, linen, rice, sugarcane, bagasse, straw, bamboo, kenaf, jute, andflax, and mixtures thereof. In some embodiments metal or inorganicfibers can also be included with the cellulosic or lignocellulosicmaterial or included in a portion of the paper product being irradiated.

In a further aspect, the invention features a method of identifyingwhether a paper product is marked. The method includes comparing thefunctionalization of a sample paper product to the functionalization ofa marked paper product.

In some cases, the method includes determining the functionalization ofthe sample paper product using infrared spectrometry (IR). The methodmay include comparing the number of carboxylic acid groups present inthe sample paper product with the number of carboxylic acid groupspresent in the marked paper product.

In some cases, the functionalization is determined using atomic forcemicroscopy (AFM), chemical force microscopy (CFM), or electron spinresonance (ESR).

The paper product may be, for example, currency or a work of art.

In any of the methods disclosed herein, functionalization can includeincreasing the number of carboxylic acid groups present in the paper.The number of carboxylic acid groups is determined by titration.

The irradiated material can also include functional groups selected fromthe group consisting of aldehyde groups, nitroso groups, nitrile groups,nitro groups, ketone groups, amino groups, alkyl amino groups, alkylgroups, chloroalkyl groups, chlorofluoroalkyl groups, and enol groups.

In some implementations, the irradiated material may include a pluralityof saccharide units arranged in a molecular chain, and from about 1 outof every 5 to about 1 out of every 1500 saccharide units comprises anitroso, nitro, or nitrile group, e.g., from about 1 out of every 10 toabout 1 out of every 1000 saccharide units of each chain comprises anitroso, nitro, or nitrile group, or from about 1 out of every 35 toabout 1 out of every 750 saccharide units of each chain comprises anitroso, nitro, or nitrile group. In some cases the irradiated materialcomprises a mixture of nitrile groups and carboxylic acid groups.

In some embodiments, the saccharide units can include substantially onlya single type of group, such as a carboxylic acid group, a nitrilegroup, a nitroso group or a nitro group.

The term “paper,” as used herein, is intended to includecellulose-containing sheet materials and composite sheet materialscontaining cellulose. For example, the paper may include cellulose in aplastic matrix, or cellulose combined with additives or binders.

In any of the methods disclosed herein, radiation may be applied from adevice that is in a vault.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although methods and materialssimilar or equivalent to those described herein can be used in thepractice or testing of the present invention, suitable methods andmaterials are described below. All mentioned publications, patentapplications, patents, and other references are incorporated herein byreference in their entirety. In case of conflict, the presentspecification, including definitions, will control. In addition, thematerials, methods and examples are illustrative only and not intendedto be limiting.

Other features and advantages of the invention will be apparent from thefollowing detailed description, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a diagrammatic view of a paper making system.

FIG. 2 is a diagram that illustrates changing a molecular and/or asupramolecular structure of a fibrous material.

FIG. 3 is a perspective, cut-away view of a gamma irradiator housed in aconcrete vault.

FIG. 4 is an enlarged perspective view of region R of FIG. 3.

FIG. 5 is a schematic diagram of a DC accelerator.

DETAILED DESCRIPTION

As discussed above, the invention is based, in part, on the discoverythat by irradiating fibrous materials, i.e., cellulosic andlignocellulosic materials, at appropriate levels, the molecularstructure of at least a cellulosic portion of the fibrous material canbe changed, changing the functionalization of the fibrous material. Inaddition to marking the paper, changing the functionalization can alsofavorably affect the surface properties of a paper product, e.g., thereceptivity of the surface to coatings, inks and dyes.

Moreover, the change in molecular structure can include a change in anyone or more of an average molecular weight, average crystallinity,surface area, polymerization, porosity, branching, grafting, and domainsize of the cellulosic portion. These changes in molecular structure canin turn result in favorable alterations of the physical characteristicsexhibited by the fibrous materials. Such changes are discussed in detailin U.S. Ser. No. 12/417,707, filed Apr. 3, 2009, the full disclosure ofwhich is incorporated herein by reference.

Radiation can be applied at one or more selected stages of thepapermaking process. In some cases, irradiation will improve thestrength and tear resistance of the paper, by increasing the strength ofthe cellulosic fibers of which the paper is made. In addition, treatingthe cellulosic material with radiation can sterilize the material, whichmay reduce the tendency of the paper to promote the growth of mold,mildew of the like.

Irradiation is generally performed in a controlled and predeterminedmanner to provide optimal properties for a particular application, byselecting the type or types of radiation employed and/or dose or dosesof radiation applied.

A low dose of ionizing radiation can be applied, for example, afterpulping and before amalgamation of the pulped fibers into a web; to thewet fiber web; to the paper web during or after drying; or to the driedpaper web, e.g., before, during, or after subsequent processing stepssuch as sizing, coating, and calendering. It is generally preferred thatradiation be applied to the web when it has a relatively low moisturecontent. In the example shown in FIG. 1, irradiation can be performedduring drying and finishing, e.g., between sizing, drying, pressing andcalendaring operations, or during post-processing, e.g., to the finishedpaper in roll, slit roll or sheet form.

As noted above, in some embodiments radiation is applied at more thanone point during the manufacturing process. For example, ionizingradiation can be used at a relatively high dose to form or to help formthe pulp, and then later at a relatively lower dose to alter thefunctionalization of the paper. If desired, high dose radiation can beapplied to the finished paper at selected areas of the paper web tocreate locally weakened areas, e.g., to provide tear zones.

As a practical matter, using existing technology, it is generally mostdesirable to integrate the irradiation step into the papermaking processeither after pulping and prior to introduction of the pulp to thepapermaking machine, after the web has exited the papermaking machine,typically after drying and sizing, or during or after processing of theweb into a final product. In some cases, a finished or existing paperproduct, such as currency, art or documents, can be irradiated to markthe product. However, as noted above, irradiation may be performed atany desired stage in the process.

Irradiating to Affect Material Functional Groups

After treatment with one or more ionizing radiations, such as photonicradiation (e.g., X-rays or gamma-rays), e-beam radiation or irradiationwith particles heavier than electrons that are positively or negativelycharged (e.g., protons or carbon ions), the paper becomes ionized; thatis, the paper includes radicals at levels that are detectable, forexample, with an electron spin resonance spectrometer. After ionization,the paper can be quenched to reduce the level of radicals in the ionizedmaterial, e.g., such that the radicals are no longer detectable with theelectron spin resonance spectrometer. For example, the radicals can bequenched by the application of sufficient pressure to the ionizedmaterial and/or by contacting the ionized material with a fluid, such asa gas or liquid, that reacts with (quenches) the radicals. Variousgases, for example nitrogen or oxygen, or liquids, can be used to atleast aid in the quenching of the radicals and to functionalize theionized material with desired functional groups. Thus, irradiationfollowed by quenching can be used to provide pulp or paper with desiredfunctional groups, including, for example, one or more of the following:aldehyde groups, enol groups, nitroso groups, nitrile groups, nitrogroups, ketone groups, amino groups, alkyl amino groups, alkyl groups,chloroalkyl groups, chlorofluoroalkyl groups, and/or carboxylic acidgroups. These groups increase the hydrophilicity of the region of thematerial where they are present. In some implementations, the paper webis irradiated and quenched, before or after processing steps such ascoating and calendering, to affect the functionality within and/or atthe surface of the paper and thereby affect the ink receptivity andother properties of the paper.

FIG. 2 illustrates changing a molecular and/or a supramolecularstructure of fibrous material, such as paper feedstock, paper precursor(e.g., a wet paper web), or paper, by pretreating the fibrous materialwith ionizing radiation, such as with electrons or ions of sufficientenergy to ionize the material, to provide a first level of radicals. Asshown in FIG. 2, if the ionized material remains in the atmosphere, itwill be oxidized, e.g., to an extent that carboxylic acid groups aregenerated by reaction with the atmospheric oxygen. Since the radicalscan “live” for some time after irradiation, e.g., longer than 1 day, 5days, 30 days, 3 months, 6 months, or even longer than 1 year, materialproperties can continue to change over time, which in some instances canbe undesirable.

Detecting radicals in irradiated samples by electron spin resonancespectroscopy and radical lifetimes in such samples is discussed inBartolotta et al., Physics in Medicine and Biology, 46 (2001), 461-471and in Bartolotta et al., Radiation Protection Dosimetry, Vol. 84, Nos.1-4, pp. 293-296 (1999). As shown in FIG. 2, the ionized material can bequenched to functionalize and/or to stabilize the ionized material.

In some embodiments, quenching includes application of pressure to theionized material, such as by mechanically deforming the material, e.g.,directly mechanically compressing the material in one, two, or threedimensions, or applying pressure to fluid in which the material isimmersed, e.g., isostatic pressing. Pressure may be applied, e.g., bypassing the paper through a nip. In such instances, the deformation ofthe material itself brings radicals, which are often trapped incrystalline domains, into proximity close enough for the radicals torecombine, or react with another group. In some instances, pressure isapplied together with application of heat, e.g. a quantity of heatsufficient to elevate the temperature of the material to above a meltingpoint or softening point of a component of the ionized material, such aslignin, cellulose or hemicellulose. Heat can improve molecular mobilityin the material, which can aid in quenching of radicals. When pressureis utilized to quench, the pressure can be greater than about 1000 psi,such as greater than about 1250 psi, 1450 psi, 3625 psi, 5075 psi, 7250psi, 10000 psi, or even greater than 15000 psi.

In some embodiments, quenching includes contacting the ionized materialwith fluid, such as liquid or gas, e.g., a gas capable of reacting withthe radicals, such as acetylene or a mixture of acetylene in nitrogen,ethylene, chlorinated ethylenes or chlorofluoroethylenes, propylene ormixtures of these gases. In other particular embodiments, quenchingincludes contacting the ionized material with liquid, e.g., a liquidsoluble in, or at least capable of penetrating into, the ionizedmaterial and reacting with the radicals, such as a diene, such as1,5-cyclooctadiene. In some specific embodiments, the quenching includescontacting the ionized material with an antioxidant, such as Vitamin E.If desired, the material can include an antioxidant dispersed therein,and quenching can come from contacting the antioxidant dispersed in thematerial with the radicals.

Other methods for quenching are possible. For example, any method forquenching radicals in polymeric materials described in Muratoglu et al.,U.S. Patent Publication No. 2008/0067724 and Muratoglu et al., U.S. Pat.No. 7,166,650, the disclosures of which are incorporated herein byreference in their entireties, can be utilized for quenching any ionizedmaterial described herein. Furthermore, any quenching agent (describedas a “sensitizing agent” in the above-noted Muratoglu disclosures)and/or any antioxidant described in either Muratoglu reference, can beutilized to quench any ionized material.

Functionalization can be enhanced by utilizing heavy charged ions. Forexample, if it is desired to enhance oxidation, charged oxygen ions canbe utilized for the irradiation. If nitrogen functional groups aredesired, nitrogen ions or any ion that includes nitrogen can be utilizedLikewise, if sulfur or phosphorus groups are desired, sulfur orphosphorus ions can be used in the irradiation.

In some embodiments, after quenching, the quenched material can betreated with one or more further doses of radiation, such as ionizing ornon-ionizing radiation, and/or can be oxidized for additional molecularand/or supramolecular structure change.

In some embodiments, the fibrous material is irradiated under a blanketof inert gas, e.g., helium or argon, prior to quenching.

The location of the functional groups can be controlled, e.g., byselecting a particular type and dose of ionizing particles. For example,gamma radiation tends to affect the functionality of molecules withinpaper, while electron beam radiation tends to preferentially affect thefunctionality of molecules at the surface.

In some cases, functionalization of the material can occursimultaneously with irradiation, rather than as a result of a separatequenching step. In this case, the type of functional groups and degreeof oxidation can be affected in various ways, for example by controllingthe gas blanketing the material to be irradiated, through which theirradiating beam passes. Suitable gases include nitrogen, oxygen, air,ozone, nitrogen dioxide, sulfur dioxide and chlorine.

In some embodiments, functionalization results in formation of enolgroups in the fibrous material. When the fibrous material is paper, thiscan enhance receptivity of the paper to inks, adhesives, coatings, andthe like, and can provide grafting sites. Enol groups can help breakdown molecular weight, especially in the presence of added base or acid.Thus, the presence of such groups can assist with pulping. In thefinished paper product, generally the pH is close enough to neutral thatthese groups will not cause a deleterious decrease in molecular weight.

Masking

In some cases it may be desirable to irradiate and/or quench only asmall area of a paper product, e.g., to create a “watermark” or toirradiate a particular symbol printed on the paper, e.g., an “E” oncurrency. In such cases, the remainder of the paper product, which is toremain unmarked, can be masked.

If only a small portion is to be irradiated, the remainder is maskedwith a radioopaque material, e.g., lead or other heavy metal. The maskshould be of sufficient thickness to prevent radiation from passingthrough, or to reduce the radiation that passes through sufficiently toprevent marking. If it is desired to mark a particular symbol, such asthe E on currency, the paper product should be in registration with themask such that the symbol to be marked is lined up with an opening inthe mask. Techniques for such masking are well known, e.g., in thesemiconductor industry.

If only a small portion is to be quenched, the remainder of the paperproduct can be masked during quenching, e.g., with a material thatinhibits contact of the paper product with the liquid or gas used inquenching.

Particle Beam Exposure in Fluids

In some cases, the paper, or its cellulosic or lignocellulosic startingmaterials, can be exposed to a particle beam in the presence of one ormore additional fluids (e.g., gases and/or liquids). Exposure of amaterial to a particle beam in the presence of one or more additionalfluids can increase the efficiency of the treatment.

In some embodiments, the material is exposed to a particle beam in thepresence of a fluid such as air. For example, particles accelerated inan accelerator can be coupled out of the accelerator via an output port(e.g., a thin membrane such as a metal foil), pass through a volume ofspace occupied by the fluid, and then be incident on the material. Inaddition to directly treating the material, some of the particlesgenerate additional chemical species by interacting with fluid particles(e.g., ions and/or radicals generated from various constituents of air,such as ozone and oxides of nitrogen). These generated chemical speciescan also interact with the material. For example, any oxidant producedcan oxidize the material.

In certain embodiments, additional fluids can be selectively introducedinto the path of a particle beam before the beam is incident on thematerial. As discussed above, reactions between the particles of thebeam and the particles of the introduced fluids can generate additionalchemical species, which react with the material and can assist infunctionalizing the material, and/or otherwise selectively alteringcertain properties of the material. The one or more additional fluidscan be directed into the path of the beam from a supply tube, forexample. The direction and flow rate of the fluid(s) that is/areintroduced can be selected according to a desired exposure rate and/ordirection to control the efficiency of the overall treatment, includingeffects that result from both particle-based treatment and effects thatare due to the interaction of dynamically generated species from theintroduced fluid with the material. In addition to air, exemplary fluidsthat can be introduced into the ion beam include oxygen, nitrogen, oneor more noble gases, one or more halogens, and hydrogen.

Cooling Irradiated Materials

During treatment of the materials discussed above with ionizingradiation, especially at high dose rates, such as at rates greater then0.15 Mrad per second, e.g., 0.25 Mrad/s, 0.35 Mrad/s, 0.5 Mrad/s, 0.75Mrad/s or even greater than 1 Mrad/sec, the materials can retainsignificant quantities of heat so that the temperature of the materialbecomes elevated. While higher temperatures can, in some embodiments, beadvantageous, e.g., when a faster reaction rate is desired, it isadvantageous to control the heating to retain control over the chemicalreactions initiated by the ionizing radiation, such as crosslinkingand/or grafting.

For example, in one method, the material is irradiated at a firsttemperature with ionizing radiation, such as photons, electrons or ions(e.g., singularly or multiply charged cations or anions), for asufficient time and/or a sufficient dose to elevate the material to asecond temperature higher than the first temperature. The irradiatedmaterial is then cooled to a third temperature below the secondtemperature. If desired, the cooled material can be treated one or moretimes with radiation, e.g., with ionizing radiation. If desired, coolingcan be applied to the material after and/or during each radiationtreatment.

Cooling can in some cases include contacting the material with a fluid,such as a gas, at a temperature below the first or second temperature,such as gaseous nitrogen at or about 77 K. Even water, such as water ata temperature below nominal room temperature (e.g., 25 degrees Celsius)can be utilized in some implementations.

Types of Radiation

The radiation can be provided, e.g., by: 1) heavy charged particles,such as alpha particles; 2) electrons, produced, for example, in betadecay or electron beam accelerators; or 3) electromagnetic radiation,e.g., gamma rays, x-rays or ultraviolet rays. Different forms ofradiation ionize the cellulosic or lignocellulosic material viaparticular interactions, as determined by the energy of the radiation.

Heavy charged particles include alpha particles, which are identical tothe nucleus of a helium atom and are produced by alpha decay of variousradioactive nuclei, such as isotopes of bismuth, polonium, astatine,radon, francium, radium, several actinides, such as actinium, thorium,uranium, neptunium, curium, californium, americium and plutonium.

Electrons interact via Coulomb scattering and bremsstrahlung radiationproduced by changes in the velocity of electrons. Electrons can beproduced by radioactive nuclei that undergo beta decay, such as isotopesof iodine, cesium, technetium and iridium. Alternatively, an electrongun can be used as an electron source via thermionic emission.

Electromagnetic radiation interacts via three processes: photoelectricabsorption, Compton scattering and pair production. The dominatinginteraction is determined by the energy of incident radiation and theatomic number of the material. The summation of interactionscontributing to the absorbed radiation in cellulosic material can beexpressed by the mass absorption coefficient.

Electromagnetic radiation is subclassified as gamma rays, x-rays,ultraviolet rays, infrared rays, microwaves or radio waves, depending onits wavelength.

Referring to FIGS. 3 and 4 (an enlarged view of region R), gammaradiation can be provided by a gamma irradiator 10 that includes gammaradiation sources 408, e.g., ⁶⁰Co pellets, a working table 14 forholding the materials to be irradiated, and storage 16, e.g., made of aplurality iron plates. All of these components are housed in a concretecontainment chamber (vault) 20 that includes a maze entranceway 22beyond a lead-lined door 26. Storage 16 defines a plurality of channels30, e.g., sixteen or more channels, allowing the gamma radiation sourcesto pass through storage on their way proximate the working table.

In operation, the sample to be irradiated is placed on a working table.The irradiator is configured to deliver the desired dose rate andmonitoring equipment is connected to an experimental block 31. Theoperator then leaves the containment chamber, passing through the mazeentranceway and through the lead-lined door. The operator mans a controlpanel 32, instructing a computer 33 to lift the radiation sources 12into working position using cylinder 36 attached to hydraulic pump 40.

Gamma radiation has the advantage of significant penetration depth.Sources of gamma rays include radioactive nuclei, such as isotopes ofcobalt, calcium, technicium, chromium, gallium, indium, iodine, iron,krypton, samarium, selenium, sodium, thalium and xenon.

Sources of x-rays include electron beam collision with metal targets,such as tungsten or molybdenum or alloys, or compact light sources, suchas those produced commercially by Lyncean Technologies, Inc., of PaloAlto, Calif.

Sources for ultraviolet radiation include deuterium or cadmium lamps.

Sources for infrared radiation include sapphire, zinc or selenide windowceramic lamps.

Sources for microwaves include klystrons, Slevin type RF sources or atombeam sources that employ hydrogen, oxygen or nitrogen gases.

In some embodiments, a beam of electrons is used as the radiationsource. A beam of electrons has the advantages of high dose rates (e.g.,1, 5, or even 10 MRad per second), high throughput, less containment andless confinement equipment. Electrons can also be more efficient atcausing chain scission. In addition, electrons having energies of 4-10MeV can have penetration depths of 5 to 30 mm or more, such as 40 mm.

Electron beams can be generated, e.g., by electrostatic generators,cascade generators, transformer generators, low energy accelerators witha scanning system, low energy accelerators with a linear cathode, linearaccelerators, and pulsed accelerators.

Electrons as an ionizing radiation source can be useful, e.g., forrelatively thin materials, e.g., less than 0.5 inch, e.g., less than 0.4inch, 0.3 inch, 0.2 inch, or less than 0.1 inch. In some embodiments,the energy of each electron of the electron beam is from about 0.25 MeVto about 7.5 MeV (million electron volts), e.g., from about 0.5 MeV toabout 5.0 MeV, or from about 0.7 MeV to about 2.0 MeV. Electron beamirradiation devices may be procured commercially from Ion BeamApplications, Louvain-la-Neuve, Belgium or from Titan Corporation, SanDiego, Calif. Typical electron energies can be 1, 2, 4.5, 7.5, or 10MeV. Typical electron beam irradiation device power can be 1, 5, 10, 20,50, 100, 250, or 500 kW. Typical doses may take values of 1, 5, 10, 20,50, 100, or 200 kGy.

Tradeoffs in considering electron beam irradiation device powerspecifications include operating costs, capital costs, depreciation anddevice footprint. Tradeoffs in considering exposure dose levels ofelectron beam irradiation would be energy costs and environment, safety,and health (ESH) concerns. Generators are typically housed in a vault,e.g., of lead or concrete.

The electron beam irradiation device can produce either a fixed beam ora scanning beam. A scanning beam may be advantageous with large scansweep length and high scan speeds, as this would effectively replace alarge, fixed beam width. Further, available sweep widths of 0.5 m, 1 m,2 m or more are available.

In embodiments in which the irradiating is performed withelectromagnetic radiation, the electromagnetic radiation can have anenergy per photon (in electron volts) of, e.g., greater than 10² eV,e.g., greater than 10³, 10⁴, 10⁵, 10⁶ or even greater than 10⁷ eV. Insome embodiments, the electromagnetic radiation has energy per photon ofbetween 10⁴ and 10⁷, e.g., between 10⁵ and 10⁶ eV. The electromagneticradiation can have a frequency of, e.g., greater than 10¹⁶ hz, greaterthan 10¹⁷ hz, 10¹⁸, 10¹⁹, 10²⁰ or even greater than 10²¹ hz. In someembodiments, the electromagnetic radiation has a frequency of between10¹⁸ and 10²² hz, e.g., between 10¹⁹ to 10²¹ hz.

One type of accelerator that can be used to accelerate ions producedusing the sources discussed above is a Dynamitron® (available, forexample, from Radiation Dynamics Inc., now a unit of IBA,Louvain-la-Neuve, Belgium). A schematic diagram of a Dynamitron®accelerator 1500 is shown in FIG. 5. Accelerator 1500 includes aninjector 1510 (which includes an ion source) and an accelerating column1520 that includes a plurality of annular electrodes 1530. Injector 1510and column 1520 are housed within an enclosure 1540 that is evacuated bya vacuum pump 1600.

Injector 1510 produces a beam of ions 1580, and introduces beam 1580into accelerating column 1520. The annular electrodes 1530 aremaintained at different electric potentials, so that ions areaccelerated as they pass through gaps between the electrodes (e.g., theions are accelerated in the gaps, but not within the electrodes, wherethe electric potentials are uniform). As the ions travel from the top ofcolumn 1520 toward the bottom in FIG. 5, the average speed of the ionsincreases. The spacing between subsequent annular electrodes 1530typically increases, therefore, to accommodate the higher average ionspeed.

After the accelerated ions have traversed the length of column 1520, theaccelerated ion beam 1590 is coupled out of enclosure 1540 throughdelivery tube 1555. The length of delivery tube 1555 is selected topermit adequate shielding (e.g., concrete shielding) to be positionedadjacent to column 1520, isolating the column. After passing throughtube 1555, ion beam 1590 passes through scan magnet 1550. Scan magnet1550, which is controlled by an external logic unit (not shown), cansweep accelerated ion beam 1590 in controlled fashion across atwo-dimensional plane oriented perpendicular to a central axis of column1520. As shown in FIG. 5, ion beam 1590 passes through window 1560(e.g., a metal foil window or screen) and then is directed to impinge onselected regions of a sample 1570 by scan magnet 1550.

In some embodiments, the electric potentials applied to electrodes 1530are static potentials, generated, e.g., by DC potential sources. Incertain embodiments, some or all of the electric potentials applied toelectrodes 1530 are variable potentials generated by variable potentialsources. Suitable variable sources of large electric potentials includeamplified field sources, e.g. such as klystrons. Accordingly, dependingupon the nature of the potentials applied to electrodes 1530,accelerator 1500 can operate in either pulsed or continuous mode.

To achieve a selected accelerated ion energy at the output end of column1520, the length of column 1520 and the potentials applied to electrodes1530 are chosen based on considerations well-known in the art. However,it is notable that to reduce the length of column 1520, multiply-chargedions can be used in place of singly-charged ions. That is, theaccelerating effect of a selected electric potential difference betweentwo electrodes is greater for an ion bearing a charge of magnitude 2 ormore than for an ion bearing a charge of magnitude 1. Thus, an arbitraryion X²⁺ can be accelerated to final energy E over a shorter length thana corresponding arbitrary ion X³⁰ . Triply- and quadruply-charged ions(e.g., X³⁺ and X⁴⁺) can be accelerated to final energy E over evenshorter distances. Therefore, the length of column 1520 can besignificantly reduced when ion beam 1580 includes primarilymultiply-charged ion species.

To accelerate positively-charged ions, the potential differences betweenelectrodes 1530 of column 1520 are selected so that the direction ofincreasing field strength in FIG. 5 is downward (e.g., toward the bottomof column 1520). Conversely, when accelerator 1500 is used to acceleratenegatively-charged ions, the electric potential differences betweenelectrodes 1530 are reversed in column 1520, and the direction ofincreasing field strength in FIG. 5 is upward (e.g., toward the top ofcolumn 1520). Reconfiguring the electric potentials applied toelectrodes 1530 is a straightforward procedure, so that accelerator 1500can be converted relatively rapidly from accelerating positive ions toaccelerating negative ions, or vice versa. Similarly, accelerator 1500can be converted rapidly from accelerating singly-charged ions toaccelerating multiply-charged ions, and vice versa.

Various methods may be used for the generation of ions suitable for ionbeams which may be used in treating the paper or the starting cellulosicor lignocellulosic materials. After the ions have been generated, theyare typically accelerated in one or more of various types ofaccelerators, and then directed to impinge on the material to betreated. Various types of accelerators and ion beam generating equipmentare described in U.S. Ser. No. 12/417,707, incorporated by referencehereinabove.

Doses

In some embodiments, irradiating (with any radiation source or acombination of sources) is performed until the material receives a doseof at least 0.05 MRad, e.g., at least 0.1, 0.25, 1.0, 2.5, or 5.0 MRad.In some embodiments, irradiating is performed until the materialreceives a dose of between 0.1 and 2.5 MRad. Other suitable rangesinclude between 0.25 MRad and 4.0 MRad, between 0.5 MRad and 3.0 MRad,and between 1.0 MRad and 2.5 MRad.

The degree of functionalization achieved is generally higher the higherthe dose.

In some embodiments, the irradiating is performed at a dose rate ofbetween 5.0 and 1500.0 kilorads/hour, e.g., between 10.0 and 750.0kilorads/hour or between 50.0 and 350.0 kilorads/hours. When highthroughput is desired, e.g., in a high speed papermaking process,radiation can be applied at, e.g., 0.5 to 3.0 MRad/sec, or even faster,using cooling to avoid overheating the irradiated material.

In some embodiments in which coated paper is irradiated, the papercoating includes resin that is cross-linkable, e.g., diacrylate orpolyethylene. In some cases, the resin crosslinks as the paper isirradiated, which can provide a synergistic effect to optimize the scuffresistance and other surface properties of the paper. In theseembodiments, the dose of radiation is selected to be sufficiently highso as to achieve the desired functionalization of the paper, i.e., atleast about 0.25 to about 2.5 MRad, depending on the material, whilebeing sufficiently low so as to avoid deleteriously affecting the papercoating. The upper limit on the dose will vary depending on thecomposition of the coating, but in some embodiments the preferred doseis less than about 5 MRad.

In some embodiments, two or more radiation sources are used, such as twoor more ionizing radiations. For example, samples can be treated, in anyorder, with a beam of electrons, followed by gamma radiation and/or UVlight having wavelengths from about 100 nm to about 280 nm. In someembodiments, samples are treated with three ionizing radiation sources,such as a beam of electrons, gamma radiation, and energetic UV light.

Identifying Marked Paper Products

Paper products that have been marked using the methods described hereinare distinguishable from similar looking unmarked paper products bydetermining the functionality of the paper. This can be accomplished,for example, by preparing an IR scan of the paper in question, using aninfrared spectrometer, and comparing the scan to a “control” IR scan ofa marked paper. For example, if the marked paper has been byfunctionalized so as to increase the number of carboxylic acid groups inthe paper, the IR scan of a paper being tested to see whether it hasbeen similarly marked should have a carboxyl peak that is substantiallythe same height as the carboxyl peak in the control IR scan.

Alternative methods of testing whether a paper has been marked or notinclude AFM, CFM, and ESR.

Paper Additives

Any of the many additives and coatings used in the papermaking industrycan be added to or applied to the fibrous materials, papers, or anyother materials and products described herein. Additives include fillerssuch as calcium carbonate, plastic pigments, graphite, wollastonite,mica, glass, fiber glass, silica, and talc; inorganic flame retardantssuch as alumina trihydrate or magnesium hydroxide; organic flameretardants such as chlorinated or brominated organic compounds; carbonfibers; and metal fibers or powders (e.g., aluminum, stainless steel).These additives can reinforce, extend, or change electrical ormechanical properties, compatibility properties, or other properties.Other additives include starch, lignin, fragrances, coupling agents,antioxidants, opacifiers, heat stabilizers, colorants such as dyes andpigments, polymers, e.g., degradable polymers, photostabilizers, andbiocides. Representative degradable polymers include polyhydroxy acids,e.g., polylactides, polyglycolides and copolymers of lactic acid andglycolic acid, poly(hydroxybutyric acid), poly(hydroxyvaleric acid),poly[lactide-co-(e-caprolactone)], poly[glycolide-co-(e-caprolactone)],polycarbonates, poly(amino acids), poly(hydroxyalkanoate)s,polyanhydrides, polyorthoesters and blends of these polymers.

If desired, various cross-linking additives can be added. Such additivesinclude materials that are cross-linkable themselves and materials thatwill assist with cross-linking of the cellulosic or lignocellulosicmaterial in the paper. Cross-linking additives include, but are notlimited to, lignin, starch, diacrylates, divinyl compounds, andpolyethylene. In some implementations, such additives are included inconcentrations of about 0.25% to about 2.5%, e.g., about 0.5% to about1.0%.

When additives are included, they can be present in amounts, calculatedon a dry weight basis, of from below about 1 percent to as high as about80 percent, based on total weight of the fibrous material. Moretypically, amounts range from between about 0.5 percent to about 50percent by weight, e.g., from about 0.5 percent to about 5 percent, 10percent, 20 percent, 30, percent or more, e.g., 40 percent.

Any additives described herein can be encapsulated, e.g., spray dried ormicroencapsulated, e.g., to protect the additives from heat or moistureduring handling.

Suitable coatings include any of the many coatings used in the paperindustry to provide specific surface characteristics, includingperformance characteristics required for particular printingapplications.

As mentioned above, various fillers can be included in the paper. Forexample, inorganic fillers such as calcium carbonate (e.g., precipitatedcalcium carbonate or natural calcium carbonate), aragonite clay,orthorhombic clays, calcite clay, rhombohedral clays, kaolin clay,bentonite clay, dicalcium phosphate, tricalcium phosphate, calciumpyrophosphate, insoluble sodium metaphosphate, precipitated calciumcarbonate, magnesium orthophosphate, trimagnesium phosphate,hydroxyapatites, synthetic apatites, alumina, silica xerogel, metalaluminosilicate complexes, sodium aluminum silicates, zirconiumsilicate, silicon dioxide or combinations of the inorganic additives maybe used. The fillers can have, e.g., a particle size of greater than 1micron, e.g., greater than 2, 5, 10, or 25 microns or even greater than35 microns.

Nanometer scale fillers can also be used alone, or in combination withfibrous materials of any size and/or shape. The fillers can be in theform of, e.g., particles, plates or fibers. For example, nanometer sizedclays, silicon and carbon nanotubes, and silicon and carbon nanowirescan be used. The fillers can have a transverse dimension less than 1000nm, e.g., less than 900, 800, 750, 600, 500, 350, 300, 250, 200, or 100nm, or even less than 50 nm.

In some embodiments, the nano-clay is a montmorillonite. Such clays areavailable from Nanocor, Inc. and Southern Clay products, and have beendescribed in U.S. Pat. Nos. 6,849,680 and 6,737,464. The clays can besurface treated before mixing into, e.g., a resin or a fibrous material.For example, the clay can be surface treated so that its surface isionic in nature, e.g., cationic or anionic.

Aggregated or agglomerated nanometer scale fillers, or nanometer scalefillers that are assembled into supramolecular structures, e.g.,self-assembled supramolecular structures can also be used. Theaggregated or supramolecular fillers can be open or closed in structure,and can have a variety of shapes, e.g., cage, tube or spherical.

Lignin Content

The paper products discussed herein can contain lignin, for example upto 1, 2, 3, 4, 5, 7.5, 10, 15, 20, or even 25% by weight of lignin. Thislignin content can be the result of the lignin present in thelignocellulosic material(s) used to manufacture the paper.Alternatively, or in addition, lignin can be added to the paper as anadditive, as mentioned above. In this case, the lignin can be added as asolid, e.g., as a powder or other particulate material, or can bedissolved or dispersed and added in liquid form. In the latter case, thelignin can be dissolved in solvent or a solvent system. The solvent orsolvent system can be in the form of a single phase or two or morephases. Solvent systems for cellulosic and lignocellulosic materialsinclude DMSO-salt systems. Such systems include, for example, DMSO incombination with a lithium, magnesium, potassium, sodium or zinc salt.Lithium salts include LiCl, LiBr, LiI, lithium perchlorate and lithiumnitrate. Magnesium salts include magnesium nitrate and magnesiumchloride. Potassium salts include potassium iodide and nitrate. Examplesof sodium salts include sodium iodide and nitrate. Examples of zincsalts include zinc chloride and nitrate. Any salt can be anhydrous orhydrated. Typical loadings of the salt in the DMSO are between about 1and about 50 percent, e.g., between about 2 and 25, between about 3 and15 or between about 4 and 12.5 percent by weight.

In some cases, lignin will cross-link in the paper during irradiation,further enhancing the physical properties of the paper.

Paper Types

Paper is often characterized by weight. The weight assigned to a paperis the weight of a ream, 500 sheets, of varying “basic sizes,” beforethe paper is cut into the size as sold to end customers. For example, aream of 20 lb, 8½×11″ paper weighs 5 pounds, because it has been cutfrom a larger sheet into four pieces. In the United States, printingpaper is generally 20 lb, 24 lb, or 32 lb at most. Cover stock isgenerally 68 lb, and 110 lb or more.

In Europe the weight is expressed in grams per square meter (gsm or justg). Printing paper is generally between 60 g and 120 g. Anything heavierthan 160 g is considered card stock. The weight of a ream thereforedepends on the dimensions of the paper, e.g., one ream of A4 (210 mm×297mm) size (approx 8.27″×11.7″) weighs 2.5 kilograms (approx 5.5 pounds).

The density of paper ranges from 250 kg/m³ (16 lb/ft³) for tissue paperto 1500 kg/m3 (94 lb/ft³) for some specialty paper. In some cases thedensity of printing paper is about 800 kg/m³ (50 lb/ft³).

The processes described herein are suitable for use with all of thesegrades of paper, as well as other types of paper such as corrugatedcardboard, paper board, and other paper products. The processesdescribed herein may be used to treat paper that is used, for example,in any of the following applications: as postage stamps; as paper money,bank notes, securities, checks, and the like; in books, magazines,newspapers, and art; and for packaging, e.g., paper board, corrugatedcardboard, paper bags, envelopes, and boxes. The paper may besingle-layer or multi-layer paper, or may form part of a laminate. Themarking can be used in commerce to indicate purchase, use, or otherevents. For example, marking can be used to “cancel” postage, or toindicate where and/or when an item was purchased.

The paper may be made of any desired type of fiber, including fiberderived from wood and recycled paper, as well as fiber derived fromother sources. Vegetable fiber materials, such as cotton, hemp, linen,and rice, can be used alone or in combination with each other or withwood-derived fibers. Other non-wood fiber sources include, but are notlimited to, sugarcane, bagasse, straw, bamboo, kenaf, jute, flax, andcotton. A wide variety of synthetic fibers, such as polypropylene andpolyethylene, as well as other ingredients such as inorganic fillers,may be incorporated into paper as a means for imparting desirablephysical properties. It may be desirable to include these non-woodfibers in paper used in special application such as for paper money,fine stationary, art paper and other applications requiring particularstrength or aesthetic characteristics.

The paper may be irradiated before or after printing.

Process Water

In the processes disclosed herein, whenever water is used in anyprocess, it may be grey water, e.g., municipal grey water, or blackwater. In some embodiments, the grey or black water is sterilized priorto use. Sterilization may be accomplished by any desired technique, forexample by irradiation, steam, or chemical sterilization.

Other Embodiments

It is to be understood that while the invention has been described inconjunction with the detailed description thereof, the foregoingdescription is intended to illustrate and not limit the scope of theinvention, which is defined by the scope of the appended claims. Otheraspects, advantages, and modifications are within the scope of thefollowing claims.

What is claimed is:
 1. A marked paper product comprising: a sheetcomprising a cellulosic material in a plastic matrix, wherein anirradiated discrete, predefined portion of a surface of the sheetcontains functional groups or a number thereof not present in the sheetprior to irradiation or in a non-irradiated portion of the cellulosicmaterial of the same surface of the sheet, and wherein the functionalgroups not present in the sheet prior to irradiation are formed byirradiation of the discrete, predefined portion with acceleratedparticles.
 2. The marked paper product of claim 1 wherein the discreteportion of the sheet comprises a number of carboxylic acid groups thatis greater than is present in the sheet prior to irradiation or in thenon-irradiated portion of the cellulosic or lignocellulosic fibrousmaterial.
 3. The marked paper product of claim 1 wherein the functionalgroups are selected from the group consisting of aldehyde groups,nitroso groups, nitrile groups, nitro groups, ketone groups, aminogroups, alkyl amino groups, alkyl groups, chloroalkyl groups,chlorofluoroalkyl groups, and enol groups.
 4. The marked paper productof claim 1 wherein the marked paper product comprises a note.
 5. Themarked paper product of claim 4 wherein the marked paper product is apaper-comprising currency.
 6. The marked paper product of claim 5wherein the currency is paper money.
 7. The marked paper product ofclaim 1 wherein the predefined portion is in the form of a symbol. 8.The marked paper product of claim 10 wherein the predefined portion isin the form of a watermark.